1. Field of the Invention
This invention relates to a method for the production of a dislocation-free silicon single crystal by the Czochralski method. More particularly, it relates to a method for the production of a dislocation-free silicon single crystal without requiring a necking operation by the Dash method.
2. Description of Related Art
The production of a dislocation-free silicon single crystal by the Czochralski method (hereinafter referred to as "CZ method") comprises placing and melting polycrystalline silicon in a crucible, immersing a seed crystal in the melt, and pulling the seed crystal and inducing growth of a single crystal. In the production of the single crystal by this CZ method, when the seed crystal is immersed in the melt at the most initial stage of the production, this seed crystal is caused to develop dislocation therein owing to the thermal shock (thermal stress) originating in the contact with the melt. The silicon single crystal to be grown thereafter transforms into a dislocated crystal unless the seed crystal is deprived of this dislocation.
The Dash method has been heretofore available as a means for expelling the dislocation generated in the seed crystal. The Dash method effects perfect removal of the dislocation from the crystal before the main body part of crystal starts growing by performing the operation of necking for decreasing the diameter of the crystal as by causing the neck part thereof, 2-4 mm in diameter, to grow at a high pulling speed in the proximity of 6 mm/min. This operation is referred to as "Dash's neck."
After the dislocation has been removed from the neck part, the main body part of crystal has the diameter thereof enlarged until a prescribed size. Thereafter, the main body part of a prescribed length is left growing, then contracted in diameter to form a tail part, and severed from the melt. The produced crystal is extracted from the pulling device.
In the method for producing the silicon single crystal by resorting to the Dash's neck operation described above, since the weight of the grown single crystal is supported by the neck part which has the weakest strength in the whole single crystal ingot continuing growth, it sometimes occurs that the neck part will accidentally break while the growth of crystal is in process and the main body part of crystal will consequently fall down into the crucible holding the melt.
With a view to curbing the occurrence of this accident, JP-A-05043379 discloses a method which comprises forming a neck part larger in diameter than the neck part which would be formed by the Dash method and meanwhile depriving the grown crystal of dislocation. By this method, the neck part is pulled at a rate in the range of 4-6 mm/min and the removal of dislocation is effected when the neck part maintains a fixed diameter in the range of 4.5-10 mm. The removal of the dislocation becomes difficult when the diameter of the neck part exceeds 10 mm.
EP-A-0747512 discloses a method which comprises causing the middle portion and the lower portion of the neck part exceeding 10 mm in diameter to grow at a rate of less than bout 4.0 mm/min until the dislocation is removed. Though this method is effective for the p.sup.+ -type silicon single crystal whose resistivity due to doping with boron does not exceed 0.1 .OMEGA..cm, it is not effective for the standard p.sup.- -type crystal or n-type crystal.
U.S. Pat. No. 5,126,113 discloses a technique for preventing the neck part from breakage by additionally providing a holding means for use in the main body part of crystal. By this technique, after the dislocation in the crystal has been removed by the Dash's neck and before the cone portion of the main body part of crystal subsequently starts growing, the crystal is provided in the lower side of the Dash's neck part with a bulge by increasing the diameter. During the growth of the main body part of crystal, a mechanical grip takes hold of the depressed portion below the bulged portion to support the main body part of crystal. In the case of this technique, the grip has the possibility of inflicting breakage on the Dash's neck part while in the process of taking hold of the crystal.
Electrochemical Society Proceedings, Vol. 97-3, p. 123 proposed a dislocation-free seed technique which avoids the Dash's neck in a diameter of 10 mm. This technique attains growth of a dislocation-free silicon single crystal by immersing a silicon seed crystal which is formed of a dislocation-free silicon single crystal having the leading end thereof pointed after the fashion of a pencil and measuring 10 mm in diameter and which has a surface free from scratch or contamination, into the melt at a rate of 3 mm/min to a prescribed length and thereafter enlarging the diameter of crystal to form the main body part of crystal without resorting to the Dash's neck. The dislocation-free silicon single crystal is not necessarily obtained based solely on the conditions which are proper for this technique. Not only the scratch and contamination of the surface but also the surface roughness has serious effect on the generation of dislocation. When the surface roughness is large, the rugged portion of the surface concentrates stress and generates dislocation consequently. Therefore, even when the seed crystal to be used is shaped like a pencil and is free of scratch or contamination and it is immersed at a rate of 3.0 mm/min as expected by this technique, it will generate dislocation if it has a large surface roughness.
When an attempt is made to obtain the dislocation-free silicon single crystal by using the rate of 3.0 mm/min for the immersion of the seed crystal as specified for this technique, this immersion must be carried out in a specific hot zone (furnace interior structure) and the elimination of dislocation is not easily attained in a rather suddenly cooling type furnace interior structure having a temperature gradient of not less than 5.0.degree. C./mm in the solid-liquid interface between the melt and the crystal, for example.